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Using a Self-assemblable Nucleating Agent to Tailor Crystallization Behavior, Crystal Morphology, Polymorphic Crystalline Structure and Biodegradability of Poly(1,4-butylene adipate) Jinjun Yang, Rong Liang, Yichun Chen, Chunqiu Zhang, Ruiling Zhang, Xiaomin Wang, Rui Kong, and Qixian Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01783 • Publication Date (Web): 23 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017

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Industrial & Engineering Chemistry Research

Using

a

Self-assemblable

Nucleating

Agent

to

Tailor

Crystallization Behavior, Crystal Morphology, Polymorphic Crystalline Structure and Biodegradability of Poly(1,4-butylene adipate) Jinjun Yang,*,† Rong Liang,† Yichun Chen,† Chunqiu Zhang,‡ Ruiling Zhang,† ∥ Xiaomin Wang,† Rui Kong,† and Qixian Chen*,§, †

School of Environmental Science and Safety Engineering, Tianjin University of Technology, 391 Binshui Xidao, Xiqing District, Tianjin 300384, China ‡

School of Mechanical Engineering, Tianjin University of Technology, 391 Binshui Xidao, Xiqing District, Tianjin 300384, China

§

School of Life Science and Biotechnology, Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, China

∥

Ningbo Hygeia Medical Technology Company, Ltd., No. 1177 Lingyun Road, High-Tech Zone, Ningbo 315040, China *

Correspondence to: J. Yang (E-mail: [email protected]) Q. Chen (E-mail: [email protected])

ABSTRACT: In this work, the decamethylenedicarboxylic dibenzoylhydrazide (DMC-DBH) was attempted as a nucleating agent (NA) to introduce into the poly(1,4-butylene adipate) (PBA) matrix. Effects of the DMC-DBH on the PBA including the crystallization kinetics, crystal morphology, polymorphic crystalline structure and biodegradation behaviors, was investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR) and scanning electron microscopy (SEM). The crystallization temperature and crystallization rate of the PBA increased upon incorporation of the DMC-DBH. This composite was characterized with distinctive shish-kebab structure. The DMC-DBH is favorable for formation of the PBA α-crystal at markedly lower Tc as compared to the neat PBA and accelerated β-to-α phase transition. The biodegradation rate of the PBA decreased upon addition of the DMC-DBH. The mechanisms on the preferential formation of the PBA α-crystal,

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accelerated phase transition and decreased biodegradation rate were also proposed and discussed.

INTRODUCTION Poly(1,4-butylene adipate) (PBA) as a representative linear aliphatic polyester characterized with melting-crystallization temperature (Tc)-dependent polymorphic crystalline structure, has been intensively explored in the utility biomedical and eco-friendly materials in respect to its biodegradability1-3. The PBA was characterized to possess α- and β-crystal structures when Tc exceeding 31 °C and Tc below 29 °C, respectively, whereas the (α + β) crystal blending was determined when 29 °C ≤ Tc ≤ 31 °C

1-3,8-13,17-24

. It was reported that both α- and β-crystal present the Maltese-cross

spherulites, nonetheless, the (α + β) crystal blending exhibits the ring-band spherulites8. Along a low heating rate (1 °C/min) or under a high annealing temperature (49 °C), the β-crystal is subject to transformation into α-crystal2,20. The α-crystal was characterized with a higher melting-crystallization temperature and crystallinity, larger crystal lattice size, and better thermal stability, but faster biodegradation as compared to the β-crystal3. Noteworthy was the lowest biodegradation rate of the (α + β) crystal blending with the ring-band spherulits3. To this respect, PBA is considered as a tempting matrix material for subsequent tailoring for pursuit of a desirable property (e.g. a targeted biodegradation profile) through fine-tuning the crystalline structure and/or crystal morphology. In respect to the efficiency, convenience, simplicity and low cost, nucleating agents (NAs) have been attempted to manipulate the crystallization kinetics, polymorphic crystalline structure and/or the biodegradation of the PBA.8-12,21-26 No peculiar crystal morphologies can be found in these PBA composites with NAs. In addition, these NAs aforementioned can hardly be dissolved in the PBA melting. One of the requirements of a viable NA is the miscibility with the polymer matrix in the melting state. The NA was postulated to possess a higher melting temperature (Tm) and crystallization temperature (Tc) than those of the polymer matrix, given that the


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polymer/NA composite could transform from the uniform melting into the liquid-solid state (phase separation) before the crystallization of the polymer matrix, consequently providing heterogeneous nuclei for the subsequent crystal growth of the polymer matrix.27 Several organic NAs (such as oxalamide,27 low-molecular-weight amide,28-30 and hydrazide31,32) have been documented to be partially or completely dissolved in the polymer melting, which facilitated further recrystallize, self-assemble during the melting processing. Noteworthy is these organic NAs that not only direct the crystal growth, but also induce particular crystal morphology (e.g., shish-kebab crystals) in semicrystalline polymers such as poly(lactide) (PLA). The crystal morphology and superstructure of these PLAs containing composition of NAs usually altered as a result of the change of the NA composition and crystallization thermal history. Owing to the significantly large interfacial surface area, and unique and compactly interconnected structure, the polymers with the shish-kebab crystals frequently exhibit outstanding performances than those with the common spherulitic crystals.29 Therefore, the control over the crystal morphology is indeed a feasible way to tailor the physical performances of polymeric materials in the industrial processing. The decamethylenedicarboxylic dibenzoylhydrazide (DMC-DBH) (its chemical structure in scheme 1) was attempted as a potent organic NA, aiming for enhancing the nucleating ability and crystallization temperature of the PLA.33 In this work, the DMC-DBH was employed to tailor the crystallization kinetics, crystal morphology, polymorphic crystalline structure, phase transition and biodegradation of the PBA. In our previous report,21,22 orotic acid (OA) and zinc phenylphosphonate (PPZn) as effective NAs were utilized to increase the crystallization temperature and shorten the crystallization time, modulate the polymorphism or biodegradation of the PBA without changing the crystal morphology of the PBA. In the present work, the DMC-DBH with the shish structure was expected to manipulate the crystal morphology (kebab) of the PBA, which probably further modulate the polymorphism and biodegradation of the PBA. To the best our knowledge, there is no relevant literature on modulation of the polymorphism and biodegradation of the PBA via tailoring its crystal morphology using the self-assembled NA at present. The



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PBA/DMC-DBH composite provides a model and informative platform to shed more light on relationship among crystal morphology, polymorphism and properties (biodegradation) of the polymorphic PBA via the self-assemble of the DMC-DBH. The mechanism on the formation of shish-kebab-like crystal of PBA and control of the polymorphic crystalline structure and biodegradation of the PBA were also discussed and proposed.

Scheme 1. The chemical structure of the decamethylenedicarboxylic dibenzoylhydrazide (DMC-DBH).

EXPERIMENTAL SECTION Materials PBA (Mw = 12 kDa) was purchased from Sigma-Aldrich Co. prior to use and purified by precipitation in ethanol, followed by vacuum drying at 40 °C. The DMC-DBH NA was supplied kindly by professor Pengju Pan in Zhejiang University, China. The lipase from Pseudomonas sp. was purchased from Sigma-Aldrich Co. (St. Louis, U.S.A.). Preparation of PBA/DMC-DBH Blend Samples The PBA was mixed with the DMC-DBH NA according to a solution casting approach. The detailed preparation procedure is described as follows. The DMC-DBH powder was dispersed in the chloroform under ultrasonic treatment for 1 h. The ratio of the DMC-DBH to the chloroform was adjusted at 0.1 g per 100 ml given that the diameter of the particles was determined to be lower than 20 µm. The PBA was added into the prepared suspension, and then well stirring. The chloroform was subjected to evaporate at room temperature. The composite film was dried at 40 ° C in the vacuum oven (approximate 50 Pa) for 24 h. In the current study, the composite film sample was referred as PBA/x% DMC-DBH, where x% stands for the weight percent of the DMC-DBH relative to the whole composite.


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Characterization The differential scanning calorimetry (DSC) curves of the following samples, including neat PBA, DMC-DBH and PBA/DMC-DBH were recorded using a NETZSCH differential DSC 214 Polyma instrument (NETZSCH, Germany), equipped with an IC70 intracooler under nitrogen gas flow (40 ml/min). The sample (5-8 mg) was placed and sealed in aluminum pans, which was further heated to 230 °C (lower than thermal degradation temperatures of the samples) for 5 min to eliminate completely the thermal history. Pertaining to the non-isothermal crystallization, the melted samples were programmed to cool to 0 °C at a rate of 10 °C/min. Pertaining to the isothermal crystallization of neat PBA and PBA/DMC-DBH, the samples were programmed to cool to the preset Tc at a cooling rate of 100 °C/min, which was held for adequate period aiming for a completed crystallization. Following the non-isothermal and isothermal crystallization, the samples were reheated to 230 °C at a rate of 10 °C/min to study the subsequent melting behaviors. XPF550C microscopy (Caikon Co. Ltd., Shanghai, China) was used for polarized optical microscopy (POM) observation. The sample was placed between two glass slides, heated to 230 °C at a rate of 10 °C, followed by 5 minutes incubation. A thin and melting layer of approximate 0.1 mm was created by subsequent press. The sample was quickly transferred to the other hot stage preset to desired isothermal temperatures. The crystal morphology was recorded. Rigaku RU-200 (Rigaku Corp., Tokyo, Japan) was used to record the wide angle X-ray diffraction (WAXD) patterns with Ni-filtered CuKα radiation (λ = 0.154 nm), worked at 40 kV and 200 mA. Bragg angles was set in the region of 5-40° with a scanning rate of 3°/min. The powder sample on the glass slide was kept at 230 °C for 3 min, and then transferred on a hot stage preset to a given Tc for isothermal crystallization. After the isothermal crystallization, the WAXD measurement was performed. Pertaining to the phase transition of the PBA, the melting sample at 230 °C on the glass slide was initially placed on a hot stage on which Tc = 0 °C to acquire the full PBA β-crystal, which was further transferred quickly into the oven where annealing temperature (Ta) of 45 °C is preset, for diverse incubation time (30, 60, and



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90 min) for the WAXD measurement. The TENSOR 37 infrared microscope (BRUKER, Germany) was utilized to record the transmission Fourier transform infrared (FTIR) spectrum with an accumulation of 32 scans and a resolution of 1 cm-1. The sample was first placed between two pieces of BaF2 slides and held at 230 °C on the hot stage for 5 min, which was immediately transferred on the other hot stage on which the given Tc had been preset. For estimation of the biodegradation, neat PBA and PBA/DMC-DBH films were fabricated using the same method as described in the WAXD measurement. A square film (1cm × 1cm × 0.1 cm) was prepared and each film was weighted and placed into a small glass bottle containing 1 ml phosphate buffer (0.1 M, pH = 7.4) with 1 mg/ml lipase. The glass bottle was shaked with a speed of 100 rpm at 37 °C. Three degradation periods (8, 16, and 24 h) were set. At 8 h interval, the sample was further transferred into a new glass bottle with the fresh lipase solution (the same volume and concentration). After the degradation, each sample was washed 3 times to remove the residual lipase with distilled water, followed by dried under vacuum at 40 °C for 24 h. Eventually, the dried sample was weighted once again for calculation of the weight loss. The results were expressed by three independent measurements. The Carl Zeiss Merlin Compact Field Emission scanning electron microscopy (FE-SEM) (Germany) was employed to observe the surface morphology.

RESULTS AND DISCUSSION Non-isothermal Crystallization by DSC Figure 1a shows the non-isothermal crystallization curves of neat PBA, DMC-DBH and PBA/DMC-DBH blends. The Tc of the PBA increased with the weight fraction of the DMC-DBH. As presented in Table 1, the crystallization temperature increased gradually from 25.9 °C (neat PBA) to 36.3 °C (PBA/2% DMC-DBH). It is noted that after blending with the PBA, the Tc of the DMC-DBH decreased from 174.1 °C (neat DMC-DBH) to 124.2 °C (PBA/2% DMC-DBH) and


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115.3 °C (PBA/1% DMC-DBH), as shown in the inset of Figure 1a, which maybe an indicative of the miscibility between the PBA and the DMC-DBH.19 When the amount of the DMC-DBH further reduces, nearly no crystallization peak of the DMC-DBH in the blends can be found (for brevity, only the crystallization peak of the PBA was shown). The detailed DSC data of neat PBA, DMC-DBH and PBA/DMC-DBH during the non-isothermal crystallization and subsequent heating process are presented in Table 1. The ∆Hc and ∆Hm are the crystallization and melting enthalpy that have been normalized. No melting peak of the DMC-DBH in these blends can be seen. The crystallinity (Xc) of the PBA was estimated via comparing the ∆Hm with the value of an infinitely large crystal (∆Hm0 = 135 J/g),22,34,35 (Xc = ∆Hm/∆Hm0 ×100%). From Table 1, the Xc of the PBA enhanced gradually with the weight fraction of the DMC-DBH. It suggests that the incorporated DMC-DBH is an effective NA of the PBA. In Figure 1b, Tm of the DMC-DBH (210.5 °C) is significantly higher than that of neat PBA (56.9 °C). Neat PBA exhibits 3 melting peaks. For neat PBA, its crystallization occurred in the temperature region 19.8-31.8 °C, where the (α + β) blend developed. The first and second melting peak of neat PBA maybe associated with the melting of initially developed β- and α-crystal, respectively.1,2 The third Tm with higher temperature (as denoted by the arrow in Figure 1b) of neat PBA can be seen, compared with the PBA component in the blends. It is probably ascribed to the PBA phase transition from β- to α-crystal during the heating process of neat PBA. Most recently, it has been reported that this phase transition should be ascribed to the combined phenomena of the melting the the β-phase followed by the recrystallization to the high-temperature α-phase.36 Two melting peaks of the PBA component in the blends maybe ascribed to the melting of originally formed α-crystal (which is a predominant crystal) and recrystallized α-crystal, respectively.20



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Figure 1. DSC curves in non-isothermal crystallization (a) and subsequent heating (b) process of neat PBA, DMC-DBH and PBA/DMC-DBH blends. Table 1. The DSC data in the non-isothermal crystallization and subsequent heating process. Cooling Sample

Tc (°C)

∆Hc (J/g)

PBA PBA/0.1%DMC-DBH PBA/0.3% DMC-DBH PBA/0.5% DMC-DBH

25.9 -28.1 a 28.2 -32.6a 33.1a -38.4a 35.5a -40.3a a 35.9 , -41.6a, PBA/1% DMC-DBH 115.3b -2.8b a 36.3 , -42.3a, PBA/2% DMC-DBH 124.2b -3.6b DMC-DBH 174.1 -29.7 a Refers to the data of the PBA component; b Refers to the data of the DMC-DBH component; –No data can be obtained.

Tm(°C)

∆Hm (J/g)

46.4, 53.5, 56.9 47.4a, 54.7a 49.3a, 54.0a 49.8a, 54.0a

31.4 34.3a 38.1a 40.1a

PBA crystallinity/ Xc (%) 23.3 25.4 28.2 29.7

49.6a, 53.5a

42.3a, –b

31.3

49.6a, 53.8a

43.2a, –b

32.0

210.5

33.9

–

Heating

Isothermal Crystallization by DSC Figure 2 shows the isothermal crystallization curves of neat PBA and the PBA component in the blends at Tc = 32 °C (a) and 35 °C (b). Considering the rapid crystallization of the PBA, in particular, in the presence of the DMC-DBH, the partial


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crystallization has occurred before reaching the lower Tc. Here, the relatively higher Tcs, 32 and 35 °C as two examples were chosen to study the effect of the DMC-DBH on the isothermal crystallization of the PBA α-crystal. Due to the extremely higher Tc of the DMC-DBH than the PBA, the crystallization of DMC-DBH has completed before decreasing the temperature to 32 and 35 °C even if at a high cooling rate. From Figure 2, it can be found that the crystallization time decreased greatly, especially in the presence of the 2% DMC-DBH. It is noteworthy that the fractioned crystallization (crystallization peak 1 and 2) of the PBA component occurred in the presence of the 0.1% DMC-DBH (denoted with the arrow) and the possible reason maybe attributed to the different nucleation mechanisms, as presented in the later section (the POM morphology of the PBA).

Figure 2. Isothermal crystallization curves of neat PBA and the PBA component in the blends at Tc = 32 oC (a) and 35 oC (b).

The Avrami equation37 is employed to analyze the effect of the DMC-DBH on the crystallization kinetics of the PBA at Tc = 32 and 35 °C. The detailed



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crystallization kinetics data of neat PBA and the PBA component in the composites are presented in Table 2 (the crystallization kinetics parameters of the PBA component in the presence of 0.1% DMC-DBH is not given). In Table 2, t1/2 is the crystallization half time, expressed as the crystallization time at which the 50% relative crystallinity reaches. n is the Avrami exponent related to the dimension of the crystal growth. k is the crystallization rate constant.38 As implied in Table 2, t1/2 decreased and k increased gradually along a rising composition of the DMC-DBH, indicating that the functional role of DMC-DBH in enhancing the crystallization rate of the PBA. Pertaining to n, its value was observed to be approximate 3 for the neat PBA. On the contrary, the value of n appeared to approach 2 for the composite in the presence of the DMC-DBH, suggesting that the growth dimension of the PBA crystal decreased in the presence of the DMC-DBH. A plausible reason for this decrease in the value of n from 3 (for the neat PBA) to 2 (for the DMC-DBH functionalized composite) may be as a consequence of the crystallization of the PBA occurring from pre-existing DMC-DBH nucleation points. The nucleation points became saturated, accordingly the crystal growth was limited and gave rise to an Avrami exponent below 3.

Table 2. The crystallization kinetics parameters of neat PBA and the PBA component in the blends at Tc = 32 and 35 °C. Tc = 32 °C Sample

Tc = 35 °C

t1/2 (min)

n

k (min-1)

t1/2 (min)

n

k (min-1)

PBA

5.1

3.1

1.1×10-2

5.8

2.9

8.2×10-3

PBA/0.3%DMC-DBH

2.3

2.2

6.8

2.6

2.3

4.2

PBA/0.5%DMC-DBH

2.1

1.9

8.2

2.3

1.9

5.4

PBA/1% DMC-DBH

1.8

2.1

10.3

2.1

1.8

9.1

PBA/2% DMC-DBH

1.6

1.8

13.3

1.9

2.1

10.2

Crystal Morphology by POM POM images of the PBA/DMC-DBH with 0.1% DMC-DBH (a) and 0.5% DMC-DBH (b) at the different temperatures are shown in Figure 3. At 230 °C, the


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PBA/DMC-DBH exhibited the melting state (see the panel a1 and b1). At 100 °C, the DMC-DBH crystallized to form the rod-like crystal and the number of the rod-like crystal increased with the weight of the DMC-DBH (panel a2 and b2). Upon further reducing the temperature to 32 °C, in the presence of the 0.1% DMC-DBH, the PBA crystal grown on the DMC-DBH surface with the growth direction perpendicular to the axis of the DMC-DBH rod to develop the interesting shish-kebab-like structure (as denoted in the ellipse part in the panel a3). Meanwhile, the PBA also formed the classic Maltese-cross spherulite22 (as denoted in the square or rectangle part in the panel a3). That is, a portion of the PBA developed via the heterogeneous nucleation with the DMC-DBH acting as the nucleation site and the other portion of the PBA probably nucleated on other unknown heterogeneities. So the fractioned crystallization in the isothermal crystallization process of the PBA/0.1% DMC-DBH at Tc = 32 °C (Figure 2a) is perhaps associated with the different NAs. However, no fractioned crystallization can be observed in the non-isothermal crystallization process (Figure 1a) of the PBA with the weight fraction of 0.1% DMC-DBH. It is probably attributed to the rapid crystallization of the PBA at Tc = 28.2 °C (Table 1) which is lower than 32 °C. In addition, two very close crystallization temperatures in the non-isothermal crystallization process resulted in the overlap of the crystallization peak. For the PBA/0.5% DMC-DBH (panel b3), only the shish-kebab-like crystal appeared and no Maltese-cross spherulite can be found. For the other blend samples at various Tcs, the PBA/DMC-DBH exhibited similar shish-kebab-like crystal. Moreover, the denser and shorter PBA/DMC-DBH crystals were observed upon the growing composition of the DMC-DBH (data now shown). In our previous reports in which two NAs (OA and PPZn) decrease the spherulite size of the PBA alone and no new crystal morphology appears for the PBA.21,22 In this case, we are motivated by the peculiar shish-kebab structure between the PBA and DMC-DBH NA to explore the formation mechanism and further investigate the effect of the crystal morphology on the polymorphism and biodegradation.



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Figure 3. POM pictures of the PBA/DMC-DBH with 0.1% DMC-DBH (a) and 0.5% DMC-DBH (b) at the different temperatures.

Intermolecular Interaction by FTIR The well-defined shish-kebab-like crystal structure of the PBA/DMC-DBH suggests that the PBA crystal (kebab) aligned with its direction perpendicular to the axis of the DMC-DBH (shish). Figure 4 presents the FTIR spectra of neat PBA, neat DMC-DBH and PBA/DMC-DBH melt-crystallized at 32 °C. The 1602 cm-1 band is assigned to –C=O group of neat DMC-DBH, which shifted to 1598 cm-1 after blending with the PBA (Figure 4a). In Fig. 4b, the 3206 cm-1 band is associated with –NH– group of neat DMC-DBH, which shifted to the 3201 cm-1 with the addition of the PBA. In the aforementioned two wavenumber ranges, no absorption peak of the PBA could be found. These low-frequency shifts of the bands attributed to the –C=O and/or –NH– would suggest the existence of the hydrogen bonding interactions. Similar phenomenon was also reported for a PLA/aryl amide NA blend pair,30 where


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the ν(C=O) (carbonyl stretching band) of PLA shifts and ν(N–H) (N–H stretching band) of N,N′,N″-tricyclohexyl-1,3,5-benzenetricarboxylamide (BTCA) shifts occur in the PLA/BTCA composite with a high content of the BTCA.

Figure 4. FTIR spectra of neat PBA, neat DMC-DBH and PBA/DMC-DBH in the range of (a) 1560-1670 cm-1 and (b) 3120-3350 cm-1.

Proposed Mechanism for Nucleation and Crystallization of the DMC-DBH/PBA Several mechanisms have been documented to illustrate the increased crystallization of the NA-nucleated polymer, such as, the epitaxial growth,39 hydrogen bonding interaction,30,31,40 chemical reaction.41 Aiming for understanding of the formation of the hydrogen bond and crystallization process of the PBA/DMC-DBH, the schematic illustration is presented in Figure 5. The hydrogen bonds forms between the nitrogen atom of the –NH– group and the oxygen atom of the –C=O group of the DMC-DBH and the hydrogen atom of the PBA. The mechanism of the nucleation and growth of the PBA on the DMC-DBH is shown in Figure 5b. The DMC-DBH first crystallizes and self-assembles into the rod-like shish for the PBA/DMC-DBH blends upon cooling from the homogeneous melting state. The hydrogen bond interaction between –NH– (and –C=O) of the DMC-DBH and –CH2 of the PBA promotes the migration of the molecular chains of the PBA to the DMC-DBH shish surface, which



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induces the subsequent ordered nucleation and epitaxial growth of the PBA kebab on the DMC-DBH shish.30

Figure 5. Schematic illustration for (a) hydrogen bond formation between PBA and DMC-DBH (b) crystallization process of the DMC-DBH-nucleated PBA.

Polymorphic Crystalline Structure and Phase Transition by WAXD Figure 6 shows the WAXD patterns of the PBA melt-crystallized at various Tcs with (a) 0% DMC-DBH (neat PBA), (b) 0.1% DMC-DBH and (c) 1% DMC-DBH. It can be seen that at Tc = 32 °C, 31~29 °C and 28 °C, neat PBA powder crystallized into the α-crystal, (α+β) blend and β-crystal, respectively, in good agreement with the previous report.1,22 After the addition of 0.1% DMC-DBH, the lowest temperature of the formation of pure PBA α-crystal (marked as Tc,α) decreased to 28 °C (with comparison with 32 oC of neat PBA) and the temperature at which the α-modification emerged (marked as Tc,e) decreased to 20 oC (with comparison with 28 °C of neat PBA), as shown in Figure 4b. With further increasing the weight fraction of the DMC-DBH to 1% (Figure 4c), the Tc,α and Tc,e decreased to 20 and 8 °C, respectively. It suggests that the DMC-DBH is favorable for the formation of the PBA α-crystal. To follow the effect of the DMC-DBH on the PBA polymorphic crystalline structure, the Tc values of the PBA α- and β-modification are plotted against a function of the


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weight fraction of the DMC-DBH, as depicted in Figure 7. It can be clearly seen that the more the amount of the DMC-DBH is, the lower the Tc,α and Tc,e of the PBA are. In particular, in the presence of 2% DMC-DBH, the Tc,α and Tc,e of the PBA decreased highly to 13 and 2 °C.

Figure 6. The WAXD patterns of the PBA powder melt-crystallized at various Tcs with (a) 0% DMC-DBH, (b) 0.1% DMC-DBH and (c) 1% DMC-DBH.

Figure 7. Tc of the PBA α- and β-crystal as a function of the weight fraction of the DMC-DBH.

Figure 8 shows WAXD patterns of neat PBA (a), the PBA component with 0.1% DMC-DBH (b) and 1% DMC-DBH (c) at various annealing periods (annealing



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temperature, Ta = 45 °C). Prior to the annealing experiment, the sample film isothermally crystallized at Tc = 0 °C to form the pure β-crystal from the melting state. Only the diffraction peaks assigned to the β-crystal could be found (the blue pattern in Figure 8a), indicating that the pure PBA β-crystal developed at Tc = 0 °C. Upon annealing at Ta = 45 °C for 30, 60 and 90 min, only a very weak peak assigned to α(110) appeared, suggesting that the phase transition from β- to α-crystal is very slow for neat PBA. With incorporation of 0.1% DMC-DBH and at ta = 30 and 60 min, the phase transition rate is still low. While at Ta = 90 min, the intensity of the α(110) peak is highly stronger than that of the β(110), suggesting that the α-crystal is the predominant crystalline phase (Figure 8b). Further increasing the weight fraction of the DMC-DBH to 1% (Figure 8c), the phase transition was significantly speeded up, as reflected by the fact that no diffraction peak associated with the β-crystal could be observable at Ta = 90 min, suggesting that the β-crystal has transformed completely into the α-counterpart. For the other blend systems, similar accelerated phase transition of the PBA could be found (data not shown). The β-to-α phase transition is mainly attributed to the rearrangement or adjustment of the PBA molecular chain. Given that the differences in the crystal lattice structure of the PBA β- and α-crystal, it was documented that there are three kinds of molecular chain motion for demonstrating the PBA phase transition, for example, shift, rotation and shrink.2,22


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Figure 8. WAXD patterns of neat PBA (a), the PBA component with 0.1% DMC-DBH (b) and 1% DMC-DBH (c) at various annealing time length (annealing temperature, Ta = 45 oC).

Proposed Mechanisms for Decreased Tc,α and Accelerated Phase Transition According the classical metastable theory,2,21,22,42,43 in the Gibbs free energy profiles, the so-called metastable phases will fall into one of the multiple local free energy minima and eventually transform into the thermodynamically stable state of minimum global free energy. Among all crystal phase, only one is in a metastable state at a specific temperature and pressure. Herein, the PBA β-modification as the metastable phase would adopt a favorable pathway to fall into the local free energy minimum for the kinetic effect.2,22 In our present work, the interface between the PBA and the DMC-DBH is favorable to enhance the nucleation in the crystallization process, probably associated with a depression in the surface free energy of the formation of the crystal nuclei near to the phase interface.21,44-48 The reduction in the surface energy facilitated the nucleation and enhanced the mobility of the PBA chain near the interface,21,49 resulting in the depression of the Tc,α. Another possible reason



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for the preferential formation of the PBA α-crystal in the presence of the DMC-DBH is the epitaxial growth of the PBA α-crystal on the surface of the DMC-DBH, attributed to good lattice matching between the DMC-DBH and PBA α-crystal. Similar reports have demonstrated that the polymorphic selective effect of the NAs on the PBA α-crystal is mainly ascribed to their perfect lattice matching.19,22,23 As reported by Ozaki et al.36, the obtained α-phase in the PBA phase transition is the recrystallized α-phase at a high temperature, not that via the solid-solid phase transition mechanism. In this work, the annealing temperature (45 °C) is favorable for the recrystallization of the PBA α-crystal, in the presence of polymorphic (α-phase) selective DMC-DBH.

Enzymatic Degradation From Figure 7, it is hardly to achieve the PBA (α + β) blend at a specific Tc for neat PBA and the PBA/DMC-DBH, so two Tcs (35 and 0 °C) were chosen to develop the pure α- and β-crystal, respectively, to investigate the effect of the polymorphism on the enzymatic degradation of the PBA. Here, the SEM images of neat PBA (a), PBA/0.5%DMC-DBH (b) and PBA/2%DMC-DBH (c) after 24 h of enzymatic degradation period, are shown in Figure 9. From Figure 9a1 and 9a2, the rough surface of the sample film indicates the PBA degradation in the presence of the lipase. From Figures 9b and 9c, more distinct shish-kebab-like structure of the PBA/DMC-DBH can be found because a part of compactly aligned PBA as the kebab along with the axis of the DMC-DBH rod (shish) was degraded by the lipase. In addition, the PBA layers rather than fibrils perpendicularly grown on the DMC-DBH shish.


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Figure 9. SEM images of neat PBA (a), PBA/0.5%DMC-DBH (b) and PBA/2%DMC-DBH (c) after 24h of enzymatic degradation time. All the images are enlarged by 2000 times.

The weight losses of neat PBA and the PBA component in the presence of the DMC-DBH after the enzymatic degradation are presented in Figure 10. In Figure 10a (the PBA α-crystal develops at Tc = 35 °C) and 10b (the PBA β-crystal develops at Tc = 0 °C), the weight loss of the PBA decreased with the weight fraction of the DMC-DBH regardless of the crystal modification. It is probably ascribed to the following two points. First, it is relatively difficult for the lipase to attack the compactly aligned PBA (see the panel a3 and b3 in Figure 3) in the presence of the DMC-DBH. Second, the DMC-DBH enhanced the crystallinity of the PBA, leading to the reduction of the weight loss of the PBA. Panel c, d and e suggest that weight loss of the PBA α-crystal is higher than that of the β-one regardless of the presence of the DMC-DBH. It was reported that, the different packing manner of molecular chain, the



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chain mobility, surface structure of lamellar crystal, crystallization mechanism and chain interaction account for the biodegradation of the polymorphic crystalline structure.3 In-depth investigation are needed.

Figure 10. The weight loss of neat PBA and DMC-DBH-nucleated PBA at (a) 35 oC (α-crystal) and (b) 0 oC (β-crystal) in the enzymatic experiment. (c), (d) and (e) show the weight loss of the PBA with various weight fractions of DMC-DBH in the degradation period of 8, 16 and 24 h, respectively.

CONCLUSIONS In the presence of the DMC-DBH, the Tc and crystallization rate increased, and the crystallization time decreased for the PBA, validating the function role of DMC-DBH as the nucleating agent. The DMC-DBH first crystallized and self-assembled into the rod-like (shish) structure, and then the PBA as the kebab grown with its growth direction perpendicular to the axis of the DMC-DBH shish, to form the shish-kebab-like structure. The DMC-DBH facilitated the formation of the α-crystal and accelerated the β-to-α phase transition of the PBA, mainly attributed to


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the reduction in the surface energy, which accelerated the nucleation and improved the mobility of the PBA chain near the interface between the PBA and DMC-DBH. The DMC-DBH decreased the biodegradation rate of the PBA, probably ascribed to the compactly gown and aligned PBA kebab structure along with the DMC-DBH shish, and the enhanced crystallinity of the PBA. In comparison with our previously investigated two NAs (OA and PPZn), the formed peculiar shish-kebab crystal structure between the PBA and DMC-DBH in this work provides more informative platform to investigate the effect of the crystal morphology on the biodegradation of the PBA. This system also provides a good example and feasible way to tailor the property of the polymeric material via manipulating its crystal morphology.

Acknowledgements This work was financially supported by the “Natural Science Foundation of Tianjin City (15JCYBJC47300)”, “National Natural Science Foundation of China (21304070)”, “Major Program of National Natural Science Foundation of China (11432016)” and “Municipal key Program of Natural Science Foundation of Tianjin (14JCZDJC40700)”.

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For table of contents only

The DMC-DBH assembled into the rod-like (shish) structure and the PBA grown with its growth direction perpendicular to the axis of the DMC-DBH shish to form the shish-kebab-like structure. The DMC-DBH not only manipulates the Tc of the PBA α-crystal and phase transition but also tailors the biodegradation rate of the PBA.


Using a Self-Assemblable Nucleating Agent To ... - ACS Publications

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